The present invention relates to the field of semiconductor based sensors for detecting particles or electromagnetic radiation. In particular improved sensor structures for detecting particles or electromagnetic radiation with a high sensitivity are disclosed as well as a sensor comprising an array of such sensor structures. The present invention also relates to a method of manufacturing such sensor structures and sensors.
Semiconductor based sensors and devices for detecting electromagnetic radiation are known in the art. Examples of such sensors are disclosed in WO 93/19489 and in EP-0739039. These sensors are implemented in a semiconductor substrate in CMOS- or MOS technology. In these sensors, the regions adapted for collecting charge carriers being generated by the radiation in the semiconductor substrate are forming a p-n or a n-p junction (diode) with the substrate that is of a n type conductivity or p type conductivity respectively. Such junctions are called collection junctions. In what follows, as an example, a p-type substrate 1 provided with an n-type collection region 2 is considered, as shown in FIG. 1.
Electromagnetic radiation 3, e.g. photons, impinging on the semiconductor pixel 4 is absorbed somewhere in the semiconductor material. By the absorption of the photon energy, an electron-hole pair is created. The majority carrier of this pair (a hole in case of the p-type semiconductor material) stays unnoticed in a sea of majority carriers already present in that material. The minority carrier 5 (in casu the electron) diffuses towards the junction 6, across the depletion layer 7 of the junction 6 and is collected by the region(s) 2 adapted for collecting charge carriers, thus realising a charge current of 1 electron.
Electrons 5 from the electron-hole pairs generated in the depletion layer 7 of the junction 6 are collected by the built-in electric field in the depletion layer 7. But also electrons 5 generated at considerable distance from the junction 6, outside the depletion layer 7, are collected. In fact all charges 5 generated within a so-called recombination length from the collection junction 6 have a chance of diffusing to that junction 6 and of being collected. Typically a depletion layer 7 is a few tenth of a xcexcm wide, while the distance over which electrons 5 (minority carriers) can diffuse, before being recombined, is typically more than 10 xcexcm. In most cases, the main detection volume of a photodiode 4 is outside the depletion layer 7.
In a pixel array 8, being an array of pixels for detection of electromagnetic radiation, in a situation where electrons 5 are generated deep in the semiconductor material, such as when generated by infrared light or X-rays, these electrons are collected by diffusion. Electrons diffuse through the semiconductor material in a random motion, as indicated by the arrows 9 in FIG. 2. This means that, during the time that electrons 5 diffuse upwards towards the junction 6, they also diffuse laterally in a random fashion. If the material is homogeneous, electrons 5 diffuse in a spherical fashion through the material, as indicated by the sphere 10. Therefore, the lateral diffusion of electrons 5 is of about the same distance as the vertical diffusion towards the junction 6. If an electron 5 is generated at a depth of e.g. 20 xcexcm, which is not unlikely e.g. for near infrared light, this electron 5 can be expected to arrive at a surface 11 of the semiconductor material within an area with a diameter of 40 xcexcm or more. This results in an apparent unsharpness in the image, as electrons 5 generated underneath a certain pixel, may be collected by another (neighbouring) pixel.
For this reason, typically homogeneous Silicon material is not suitable for the manufacturing of image sensors. In order to reduce the lateral diffusion of charges, epitaxial wafers 12 are used, as illustrated in FIG. 2. The bulk 13 of these wafers 12 is highly doped (e.g. highly doped p-type Silicon), so that no electrons 5 can exist in there (they almost immediately recombine with free holes), and they have a thin, e.g. 5 xcexcm, lowly doped top layer (epitaxial layer 14). As this top layer 14 is thin, the vertical diffusion can happen over a short distance only (5 xcexcm), and therefore the lateral diffusion can never become large. Nevertheless, for very small pitch pixels, e.g. 4 xcexcm or even 3 xcexcm, the lateral diffusion may be again in the order of the pixel pitch or larger, thus resulting again in unsharpness.
Roughly it can be stated that the unsharpness created by diffusion is about equal to the thickness of the epitaxial layer 14. Free electrons 5 will not penetrate into the bulk wafer 13, as this bulk wafer 13 is highly doped, and therefore there is an electrostatic barrier between the epitaxial layer 14 and the bulk wafer 13. Free electrons 5 will diffuse through the epitaxial layer 14 until one of the following conditions occurs:
(a) recombination, i.e. the free electron 5 meets a free hole, and they recombine into a bound electron, or
(b) the electron 5 is caught by an n-type junction 6 (in case of a p-type substrate).
Of course only the second one of these events is of interest for the detection of electromagnetic radiation 3.
The junction 6 in the epitaxial layer 14 thus seems to have an inherent sharpness of about the same value as the thickness of that epitaxial layer 14. In order to make small pixels which are still sharp, in theory wafers with a thinner epitaxial layer 14 can be used. However, this shows the following disadvantages:
the thinner epitaxial layer reduces the photosensitive volume, which decreases the sensitivity, especially in the yellow, red and near infrared wavelengths, and
the state of the art CMOS processes employ bulk wafers 12 or epiwafers with 5 xcexcm thickness of the epitaxial layer 14; 4 xcexcm and even 3 xcexcm may be envisaged, but less is not compatible with CMOS processing, as the well- and tub-implants reach beyond 2 xcexcm thickness, and in that case would touch the highly doped bulk Silicon material 13.
It is an object of the present invention to provide a sensor structure for detecting high energy particles or electromagnetic radiation, which has a good sensitivity, thus which has a sensitive volume which is large enough, and which does not present problems with deep implants touching the highly doped bulk material.
It is another object of the present invention to provide a sensor structure for detecting high energy particles or electromagnetic radiation, which has an increased sharpness while keeping the same sensitive volume as used in the prior art, or to provide a sensor structure for detecting particles or electromagnetic radiation with drastically increased sensitive volume while maintaining the same sharpness associated with a much shallower epitaxial layer as known in the prior art.
It is also an object of the present invention to provide an array of sensor structures for detecting high energy particles or electromagnetic radiation with increased sharpness.
It is yet a further object of the present invention to provide a method to obtain sensor structures for detection of high energy particles or electromagnetic radiation, these sensor structures having an increased sharpness.
The high energy particles to be detected are for example particles, such as xcex1-particles, xcex2-particles, xcex3-particles, nuclear or sub-nuclear particles. Electromagnetic radiation to be detected is for example cosmic rays, X-rays, ultraviolet light, infrared light and visible light.
The above objectives are accomplished by the devices and methods according to the present invention.
In first instance, the present invention provides a particle or electromagnetic radiation sensor structure, comprising a substrate having a major surface and a sensitive layer on the major surface of the substrate, the sensitive layer being sensitive to particle or electromagnetic radiation and having a first surface remote from the substrate. The sensitive layer has a doping concentration gradient from a higher doping level to a lower doping level, the doping concentration decreasing from the substrate to the first surface of the sensitive layer such that over any distance across the sensitive layer which is half of the thickness of the sensitive layer, the ratio between the highest doping level and the lowest doping level is at least a factor 2, preferably at least a factor 3 or more.
The thickness of the sensitive layer may at least be 3 microns.
According to a first embodiment, the sensitive layer comprises a plurality of sub-layers. This plurality of sub-layers may be epitaxial sub-layers. Each of those sub-layer may have a different doping concentration.
According to a second embodiment, the sensitive layer consists of a single layer with decreasing doping concentration.
The sensitive layer may have a continuously or gradually decreasing doping concentration or a stepwise decreasing doping concentration. The stepwise decreasing doping concentration may decrease step-wise monotonically.
In second instance, the present invention provides an array of sensor structures for detecting particles or electromagnetic radiation. Such an array comprises a plurality of sensor structures, and each sensor structure comprises a substrate having a major surface, and a sensitive layer on the major surface of the substrate, the sensitive layer being sensitive to particle or electromagnetic radiation and having a first surface remote from the substrate. The sensitive layer has a doping concentration gradient from a higher doping level to a lower doping level, the doping concentration decreasing from the substrate to the first surface of the sensitive layer. According to an embodiment, over any distance across the sensitive layer which is half of the thickness of the sensitive layer, the ratio between the highest doping level and the lowest doping level is at least a factor 2, preferably at least a factor 3 or more.
The thickness of the sensitive layer may at least be 3 microns.
According to a first embodiment, the sensitive layer comprises a plurality of sub-layers. This plurality of sub-layers may be epitaxial sub-layers. Each of those sub-layer may have a different doping concentration.
The sensitive layer may have a continuously or gradually decreasing doping concentration or a stepwise decreasing doping concentration. The stepwise decreasing doping concentration may decrease step-wise monotonically.
The present invention also provides a MOS based pixel structure for detecting light, comprising a plurality of sensor structures according to the present invention, or an array of sensor structures according to the present invention.
The present invention furthermore provides a method to obtain sensor structures for detection of particles or electromagnetic radiation. This method comprises the steps of:
providing a substrate with a major surface,
on the major surface of the substrate, providing a sensitive layer having a first surface remote from the substrate, the sensitive layer being sensitive to particles or electromagnetic radiation, and
providing the sensitive layer with a doping concentration gradient, the doping concentration decreasing from the substrate to the first surface of the sensitive layer.
The step of providing the sensitive layer may be such that over any distance across the thickness of the sensitive layer which is half of the thickness of the sensitive layer, the ratio between the highest doping level and the lowest doping level is at least a factor 2, preferably at least a factor 3 or more.
The step of providing a sensitive layer may either comprise providing a single sensitive layer, or providing a stack of a plurality of sub-layers. The sub-layers may be epitaxial layers. Each of the sub-layers may be provided with a different doping concentration.